JUBA, South Sudan – A top UN official says he is deeply shocked at the killing and harassment of aid workers in South Sudan.

Stephen O'Brien, U.N. under-secretary-general for humanitarian affairs and emergency relief coordinator, said Saturday that 27 aid workers had died since the conflict began in South Sudan. He says South Sudan's government has given assurances that it will support investigations into deaths of the humanitarian workers. He asked the groups in the conflict uphold their obligations to comply with the principals of international humanitarian law.

South Sudan's civil war, which started in December 2013, has inflamed the country's ethnic tensions. The followers of President Salva Kiir, mostly from the Dinka ethnic group, are pitted against the Nuer of former vice president Riek Machar in a fight that started in December 2013.